Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs) using backlighting, and a wider spectrum of colors of emitted light in full-color OLED displays.
Applications of OLED devices include active matrix image displays, passive matrix image displays, and area lighting devices such as, for example, selective desktop lighting devices. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device.
The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
In top-emitting OLED devices, light is emitted through an upper electrode or top electrode which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s) can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Consequently, the lower electrodes (anodes) can be formed over relatively complex drive circuitry in an active matrix OLED image display. Top-emitting OLED displays offer the potential to improve display performance compared with bottom-emitting OLED displays by:                1) increasing the aperture ratio, therefore permitting pixels of the display to operate at a lower current density which results in improved operational stability;        2) permitting more complex drive circuitry to enable improved control of pixel current, leading to enhanced display uniformity and to improved display stability;        3) enabling the use of lower mobility materials, e.g. amorphous silicon, to be considered in forming the thin-film transistor (TFT) drive circuitry; and        4) permitting incorporation of elements which can increase the out-coupling of light generated within the organic EL medium structure to provide increased efficiency of emitted light.        
However, bottom-emitting OLED devices continue to find widespread use in displays of data or in the field of advertising.
Unprotected OLED display devices, irrespective of device configuration, are prone to relatively rapid degradation of performance due to adverse effects of moisture and/or oxygen present in the ambient environment. Additionally, unprotected devices can be subject to mechanical damage caused by abrasion. Various efforts have been directed at providing packaged OLED displays in which the packaging approaches offer improved operational lifetime of displays which is, however, still limited so that widespread adoption of OLED display devices is currently restricted.
Included in these efforts at providing packaged OLED devices or displays are cover plates which are adhesively bonded over an upper surface of an OLED device. Adhesive bonding of a cover plate has been provided in prior art packaging approaches by either forming a perimeter seal for bonding a cover plate along a device perimeter, or by uniformly bonding a cover plate over an entire device area. Typically, such cover plate bonding has been achieved by dispensing a flowable adhesive material on the cover plate or on the upper surface of the OLED device, bringing the cover plate and the device surface in contact, followed by curing the adhesive material by a thermal curing process or by a radiation curing process.
Representative descriptions of such prior art cover plate bonding approaches are provided in U.S. Patent Application Publications 2002/0187775 A1 by Maruyama et al.; 2002/0193035 A1 by Wei et al.; 2002/0155320 A1 by Park et al.; and commonly assigned commonly assigned U.S. patent application Ser. No. 10/759,914 filed Jan. 16, 2004 by Yokajty et al., entitled “Method of Making an OLED Display Device With Enhanced Optical and Mechanical Properties”, the disclosure of which is herein incorporated by reference.
Maruyama et al. propose a perimeter seal which is formed between two concave grooves disposed near perimeter areas of an OLED device. The perimeter seal provides a spacing between a device surface and a surface of a second substrate which functions as a cover plate. This spacing can be filled with an inert gas. Maruyama et al. do not suggest or disclose electrical interconnect areas, nor approaches to keep such interconnects free from perimeter seal material.
Wei et al. disclose a package method and apparatus for organic electroluminescent display. A certain amount of an ultraviolet curing resin or thermal curing resin is spread on a lamination plate or a substrate. A trench is formed at an edge of the lamination plate. Upon aligning the lamination plate with the substrate, the space between the lamination plate and the substrate is controlled by adjusting lamination pressure so that excess resin flows into the trench at the edge of the lamination plate, and the dimensions of the package can be controlled. The resin is cured by ultraviolet radiation or by a thermal process. Thus, Wei et al. provide uniform bonding between the substrate and the lamination plate which functions as a cover plate. Wei et al. do not suggest or disclose electrical interconnect areas nor approaches to keep such interconnects free from resins.
Park et al. disclose a package method and apparatus for organic electroluminescent display. A trench is disposed on at least one of the cover plate or device substrate to prevent perimeter sealing material from contacting the display area of the OLED device. During pressing of the cover plate to the substrate, excess perimeter sealing material resin flows into the trench, and the sealing material is prevented from contacting the display area. Park et al. do not suggest or disclose electrical interconnect areas nor approaches to keep such interconnects free from perimeter sealing material.
While the perimeter seals of Maruyama et al. and of Park et al. can provide improved moisture protection, the lack of a structural buffer layer between the OLED device surface and a lower surface of the cover plate can cause mechanical and optical problems. Mechanical problems include excessive stress to the perimeter seal caused by thermal expansion and contraction under normal device operating conditions leading to leakage of the perimeter seal. Expansion of the gas in the space between the OLED device surface and the lower surface of the cover plate can lead to breakage of the device substrate or cover plate when subjected to lowered environmental pressure, especially for larger-sized displays. Optical problems include undesirable reflective or refractive optical effects at both surfaces of a transparent cover plate which is used in a top-emitting OLED display device.
Serbicki et al. recognized the importance of keeping at least outermost portions of electrical interconnect areas of OLED devices free from a flowable adhesive material. Various configurations of flow-preventing patterns are disclosed which are oriented with respect to a plurality of OLED devices on a device substrate so that flowable adhesive material is prevented from spreading into and beyond these patterns while permitted to spread uniformly over at least the display areas of the OLED devices. Upon curing of the adhesive material, a uniform structural buffer layer serves to uniformly bond a cover plate over an encapsulated surface of a pixelated OLED device while keeping the electrical interconnect areas free from adhesive material.
U.S. Pat. No. 6,268,695, assigned to Battelle Memorial Institute, describes an environmental barrier for an OLED in which a glass cover plate is not used. In this invention, the foundation is coated with three layers: a first polymer layer; a ceramic layer; and a second polymer layer. These layers are substantially transparent to the light emitted by the OLED. This invention creates an environmental barrier for an OLED display, but does not provide mechanical protection for the OLED display, especially from pressure points such as those created when a user touches the surface of the display with his or her finger.
Other effective barrier layers against moisture penetration and/or oxygen penetration into a top-emitting OLED device include a transparent encapsulation layer which can be formed by know thin-film deposition methods such as, for example, thermal vapor deposition, sputter deposition, or atomic layer deposition. Materials particularly suitable as encapsulation layer material include aluminum oxide (Al2Ox), silicon nitride (SiN), silicon-oxinitride (SiOxN1-x), and tantalum oxide (TaOx).
Due to the structure of the thin-film encapsulation layer, they do not provide adequate mechanical protection. For a top-emitting OLED device, a transparent cover plate is required to ensure mechanical protection. However, conventional perimeter sealing of the cover plate to the OLED display substrate results in the aforementioned mechanical and optical problems.
In manufacturing OLED display devices, a plurality of devices are typically manufactured on a device substrate, and are subsequently singulated or cut and separated from the device substrate. Each OLED display device includes a pixelated display area and an electrical interconnect area which is used to connect the singulated OLED display device to external electrical power and control electronics.
Irrespective of the configuration of environmental protection elements, such as an encapsulation layer and a perimeter-sealed cover plate, an encapsulation layer and a uniformly bonded cover plate, or just a perimeter-sealed cover plate, it is important to keep at least the outermost portions of the electrical interconnect area(s) free of encapsulation layer material and of sealing material or adhesive material to ensure reliable electrical connections to the interconnect area or areas.
Flowable adhesive materials are used in the above referenced U.S. Patent Applications to provide bonding between an OLED device and a cover plate either in the form of a perimeter seal or in the form of a uniform bond. The flowable adhesive material has to be dispensed in a measured amount, and curing of the spread adhesive material is required to provide effective bonding.
Dispensing of a flowable adhesive material, or of substantially viscous adhesive materials, requires a dispensing apparatus and may require a precision platform which can be translated along an x-direction and a y-direction if the adhesive is to be dispensed in a pattern. Since the organic EL medium structure of OLED devices is subject to degradation upon exposure to ultraviolet curing radiation or upon exposure to curing temperatures called for in thermally cured adhesives, attention has to be paid to curing conditions so as to avoid degrading the EL medium structure.
Therefore, it would be an advantage to provide a “dry” process of uniformly bonding a commonly shared cover plate over a plurality of encapsulated OLED devices formed on a device substrate. Preferably, curing of a bonding adhesive layer should not be required.
McCormick et al. in U.S. Patent Application Publication 2003/0143423 A1 disclose an organic electronic device which is encapsulated at least in part by an adsorbent-loaded transfer adhesive. The adsorbent may be a desiccant and/or a getterer. The adsorbent-loaded transfer adhesive may form a gasket around the device periphery, or may cover the entire device and its periphery. An encapsulation lid covers the device and the lid is adhered to a device substrate by the adhesive. The transfer adhesive is selected to be permeable to one or both of air and water vapor so that the adsorbent material loaded into the adhesive can getter oxygen and/or adsorb water vapor. All but one of the transfer adhesive materials described by McCormick et al. require either UV-curing, thermal curing, or heating the device during application of a hot-melt adhesive material. A conventional pressure-sensitive adhesive material obviates the need for UV-curing, or for subjecting an OLED device to a thermal process at a temperature and for a duration which can result in degrading one or all of the thin layers comprising the organic EL medium structure.